The radioimmunoassay for relaxin in plasma will be used as an index of relaxin secretion in the reproductive processes of the male and female, sheep and human. Particular emphasis will be placed on its relationship with other hormones, prolactin, FSH, LH, estrogen and progesterone. The radioimmunoassay will be used for fractionation and isolation of high (18,000) and low (6,000) molecular weight immuno-reactive relaxins from ovaries, uteri and placentae. Purification amino-acid sequence and bioassay will be performed on these in order to establish their relationships as either precursor/prohormone-hormone-metabolite. Radioreceptors for relaxin(s) will be surveyed to determine possible target tissues of ovarian, uterine and placental relaxin(s).